mamba
Legend
yes, but that is trivial enough, and once you add bonuses the roll under ‘falls apart’, either you have to subtract the bonus from the roll (addition is easier than subtraction) or you have to add it to the value you need to stay under rather than the roll, and that is a little weird.You need to do a slight mathematical translation to figure out the odds for roll-high.
If your target number is 15, on roll under the odds are just the number times 5%, assuming that matching the number is a success.
If your target number is 15, on roll over the odds are (21-target) times 5%, assuming that matching the number is a success.
So all in all I consider that a wash